<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <meta http-equiv="X-UA-Compatible" content="ie=edge">
  <title>树UI</title>
  <style>
  html, body {
    width: 100%;
    height: 100%;
    padding: 0;
    margin: 0;
  }

  .tree, .tree ul {
    list-style: none;
  }

  .tree li::before {
    color: #999;
    content: '';
    display: inline-block;
    width: 0;
    height: 0;
    border-style: solid;
    border-width: 6px 10.4px;
    border-color: transparent;
    border-left-color: currentColor;
    transform: translateX(6px);
  }

  .tree li.expand::before {
    transform: rotate(90deg) translateX(6px) ;
  }

  .tree li > ul {
    display: none;
  }

  .tree li.expand > ul {
    display: block;
  }
  </style>
</head>
<body>
  <ul class="tree">
    <li>项目1</li>
    <li>项目2</li>
    <li class="expand">项目3
      <ul>
        <li>子项3.1</li>
        <li>子项3.2</li>
        <li>子项3.3</li>
      </ul>
    </li>
    <li>项目4
      <ul>
        <li>子项4.1</li>
        <li>子项4.2</li>
      </ul>
    </li>
    <li>项目5</li>
  </ul>
</body>
</html>